🐛 fix: Resolve MeiliSearch Startup Sync Failure from Model Loading Order (#12397)

* fix: resolve MeiliSearch startup sync failure from model loading order

`indexSync.js` captures `mongoose.models.Message` and
`mongoose.models.Conversation` at module load time (lines 9-10).
Since PR #11830 moved model registration into `createModels()`,
these references are always `undefined` because `require('./indexSync')`
ran before `createModels(mongoose)` in `api/db/index.js`.

Move the `require('./indexSync')` below `createModels(mongoose)` so
Mongoose models are registered before `indexSync.js` captures them.

* fix: defer model lookups in indexSync, add load-order regression test

- Move mongoose.models.Message and mongoose.models.Conversation lookups
  from module-level into performSync() and the indexSync() catch block,
  eliminating the fragile load-order dependency that caused the original
  regression
- Add guard assertion that throws a clear error if models are missing
- Add comment in api/db/index.js documenting the createModels ordering
  constraint
- Add api/db/index.spec.js regression test to prevent future re-introduction
  of the load-order bug

* fix: strengthen regression test to verify call order, add guard in setTimeout path

- Rewrite index.spec.js to track call sequence via callOrder array,
  ensuring createModels executes before indexSync module loads
  (verified: test fails if order is reversed)
- Add missing model guard assertion in the setTimeout fallback path
  in indexSync.js for consistency with performSync

---------

Co-authored-by: Danny Avila <danny@librechat.ai>

api/db/index.js

@@ -1,8 +1,13 @@
 const mongoose = require('mongoose');
 const { createModels } = require('@librechat/data-schemas');
 const { connectDb } = require('./connect');
-const indexSync = require('./indexSync');
 
+// createModels MUST run before requiring indexSync.
+// indexSync.js captures mongoose.models.Message and mongoose.models.Conversation
+// at module load time. If those models are not registered first, all MeiliSearch
+// sync operations will silently fail on every startup.
 createModels(mongoose);
 
+const indexSync = require('./indexSync');
+
 module.exports = { connectDb, indexSync };